Medical imaging refers to a process through which medical professionals may visually evaluate an area of a patient's body that is not externally visible. Two common methods of medical imaging are Computed Tomography (commonly referred to as a “CT Scan” or a “CAT Scan”) and Medical Resonance Imaging (“MRI”). A MRI system uses powerful magnets and radio frequency pulses to controllably excite hydrogen nuclei in water molecules of a patient's tissue. As hydrogen protons within the tissue relax after the exciting pulse, they emit energy which is generally received by a sensitive coil within the MRI system. Tissue of varying composition will have varying hydrogen proton relaxation rates. The MRI coil will detect these differing rates, and thus, differing tissue compositions, by mathematically analyzing the emitted energy data over time. This mathematical data is converted via the use of a Fourier transform into an image or slice with received data points of differing energy rates having different contrast levels on the image. Depending on the setup of the MRI system, different tissue will have different contrast levels.
Medical imagining systems, and MRI systems in particular, are large, complex, and expensive. As a result, industry has found it beneficial to provide mobile medical imaging units that may be transported, shared, and/or leased by healthcare entities unable or unwilling to purchase a permanently installed medical imaging system. Mobile medical imaging units are typically stored in and transported by a large truck trailer. These truck trailers may be moved from location to location, used as full-service medical imaging facilities, and moved again. These truck trailers generally have an interior cabin which houses the medical imaging systems. The interior cabin may be separated into various rooms or sections. A medical imaging trailer will generally have at least one built-in scan room which allows patients to enter the trailer to complete their medical imaging procedure. Thus, the medical imaging device is not only transported by the trailer, but also operates within the trailer. The environment of the trailer, however, presents certain challenges that plague conventional medical imaging trailers.
Both the exterior environment of a medical imaging trailer and the interior environment of the trailer present design challenges. More specifically, the interaction between the exterior environment of the trailer and the interior environment of the trailer present healthcare affecting design challenges. These design challenges especially affect medical imaging trailers containing MRI devices. A first challenge involves controlling the radio frequency (“RF”) integrity of the trailer-located scan room. Since RF coils inside the magnet are used to construct a scanned image, it is important that the transmission of RF noise or RF interference (“RFI”) is prevented from entering the interior of the medical imaging trailer. Exterior RF noise may detract from the quality of the medical imaging scans conducted within the trailer. Similarly, emissions from the inside of the trailer to the exterior environment should be minimized to prevent interference with external electromechanical devices which are often sensitive medical and/or communications devices associated with the medical imaging trailer. Because RF noise is a design concern of a medical imaging trailer, RF shielding may be provided to the medical imaging trailer.
A second challenge presented by mobile imaging is the mechanical stability of the trailer. Unlike a stationary medical imaging site within a hospital or other permanent healthcare structure, a medical imaging trailer must be mobile within certain size and weight constraints (e.g., size and weight constraints specified by various government agencies, such as departments of transportation, etc.). A medical imaging trailer must therefore meet these constraints while providing a chassis or frame of sufficient rigidity and stability during the transportation stage and while scanning at a patient care site. Lack of stability, vibration, and excessive “g” loading during transportation can damage the magnet and computer equipment. During operation, the MRI scan is highly sensitive to non-damaging vibrations (e.g., vibrational frequencies provided by motors, compressors, pumps, etc.) and these vibrations may affect medical image quality. Therefore, the trailer usually includes components such as air ride suspension components, a solid chassis, solid structural frames, vibration damping parts, vibration damping landing legs and stabilizing stands, etc.
A third challenge presented to medical imaging trailers, particularly MRI imaging trailers, is the magnetic shielding of the trailer and the shielding structure's possible affects on image quality. No magnetic shielding is often preferred by MRI device manufacturers, but due to the challenging mobile environment of a medical imaging trailer and the strength of the magnetic fields, it is practical to include magnetic shielding with medical imaging trailers. Magnetic shielding, usually in the form of steel shielding applied to the walls of the trailer, is designed to contain magnetic gauss fields within a specified distance from the iso-center of the MRI magnet. Here again, not only does shielding prevent image distortions potentially caused by external magnetic fields during scanning, but also shields the exterior environment from the strong magnetic fields caused by the magnets within the MRI machine. While magnetic shielding on an MRI trailer is meant to provide better quality images when operating in the relatively challenging mobile environment, it has been discovered that sometimes the magnetic shielding of an MRI trailer may actually cause some image distortions. The Applicants believe that the relationship of the magnetic shielding structure, both in proximity and relative stability, to the iso-center of the magnet of the MRI machine may affect scanned image quality. The Applicants also believe that typical MRI trailer-based systems may not perform to the standards provided by most fixed site MRI systems for at least this reason. The Applicants believe that the scanned image quality of a trailer-based MRI system may be affected by the steel shielding of the trailer in at least three ways: temperature variations on the surfaces of the walls and roof of the trailer may cause expansion and/or contraction of the shielding; wind pressure loads applied to the walls of the trailer may move the shielding; and, the steel shield may move because of other vibrations and movements affecting the exterior structure of the trailer.
Regarding temperature variations on the surfaces of the walls and roof of the trailer, it was determined that exterior weather conditions could cause unwanted MRI image artifacts and image distortions. As the surface temperature of conventional MRI trailer walls and roof changed, the walls and roof experienced thermal expansion and contraction. While these variations could occur on a daily basis, they could also occur more frequently, for example, as clouds shifted to block or expose the sun. If the magnetic shield was fastened, coupled, or otherwise in transferable contact with the exterior structure of the trailer, the contraction and expansion of the exterior structure could affect the image to a significant and undesirable extent. Regarding wind load pressure of the exterior wall, it was determined that wind could cause sufficient movement of the exterior wall and coupled magnetic shield to also cause image artifacts and distortions. Regarding stability of the steel shield, it was determined that vibrations and or other movements of the magnetic shield could cause image artifacts and distortions.
There is a need for a medical imaging trailer capable of providing consistently high image quality during changing exterior temperatures, wind pressures, and other instability causing events without greatly increasing the weight and cost of the medical imaging trailer. More specifically, there is a need for a medical imaging trailer design having a stable wall structure with proper insulation, proper RF shielding, and a magnetic shielding structure substantially isolated from movement causing events of the outside environment.
It would be desirable to provide a medical imaging trailer having any one or more of these or other advantageous features.